1. Field of the Invention
This invention relates generally to a method for controlling power into and out of a battery pack and, more particularly, to a method for controlling charging power into and discharging power out of a battery pack for an electric or hybrid vehicle, which monitors root mean square current integrated over time and regulates power if a current threshold is exceeded.
2. Discussion of the Related Art
Electric vehicles and gasoline/electric hybrid vehicles are rapidly gaining popularity in today's automotive marketplace. Electric and hybrid vehicles offer several desirable features, such as reduced emissions, reduced usage of petroleum-based fuels, and potentially lower operating costs. A key component of both electric and hybrid vehicles is the battery pack. Battery packs in these vehicles typically consist of numerous interconnected cells, which can deliver a lot of power on demand. The battery pack also represents a substantial portion of the vehicle's cost. In order to maximize vehicle driving range and maximize battery pack life, the current to and from the battery pack must be controlled so that the battery pack does not charge or discharge too quickly, which can lead to overheating damage and other problems.
Various methods of monitoring battery packs to prevent overheating have been developed and implemented. These methods include monitoring temperature in the battery pack to detect an over-temperature condition, and monitoring state of charge of the battery pack to determine if the battery pack is being charged or discharged too quickly. However, these methods both have disadvantages. Temperature in a battery pack can only economically be monitored at a few discrete locations within the battery pack; therefore, there may be places in the battery pack which are hotter than the temperature sensors indicate, in which case it is possible that some cells in the battery pack are already damaged by the time a high temperature is detected. Also, there is a time lag involved in the heating of the battery pack, which creates a risk that power-regulating action will only be taken after some damage is already done. State of charge monitoring is typically done for every cell in the battery pack, but state of charge measurement can have a cumulative error, which makes it difficult to accurately determine whether some battery pack cells are about to experience damage. State of charge, like temperature, is also an after-the-fact indicator.
In order to more effectively protect battery packs from damage, a closed-loop, predictive monitoring and control method is needed. A method which monitors charging and discharging current over a certain time window can be used to proactively determine if power needs to be regulated in order to prevent damage to battery pack cells, thus overcoming the limitations of existing methods. Such a method can also be implemented in a way that improves the drivability of the vehicle, while still preserving the durability of the battery pack.